Our earth is called by many names. Some of them ‘Living Planet’, ‘Unique Planet’, ‘Water Planet’,’Blue Planet’.
Size of the earth : Distribution of land and water bodies:-
The earth is the fifth largest planet in the Sun’s family. The diameter of the Earth is approxomately 4 times graeter than the Moon and it is around 107 times less than that of the Sun.
The total geographical area of the Earth is 510 million sq kms of which 361 million sq kms (70.78%) is covered by water and 149 million sq kms(29.22%) is covered by land. Thus the Earth has unequal distribution of land and water bodies is 1:2.43.
The Earth’s shape is often described as Geoid, which literally means ‘Earth shaped’, or ‘Oblate sheroid’. The Earth is flattened at the poles and bulges at the equator. The Equatorial diameter of the earth is 12756 kms and Polar diameter is 12714 kms Equatorial circumference-40076 kms and Polar circumference-40008 kms. The difference of 42 kms in diameter is the proof for regarding the Geoid shape of the Earth.
The land and water bodies are unevenly distributed between the Northern and the Southern henispheres. The Northern Hemisphere has 60% of land and 40% of water. Therefore it is called the ‘Land Hemisphere’.On the other hand there is 81% of water and 19% of land in the Southern Hemisphere and so it is called the ‘Water Hemisphere’. Lattitudes and Longitudes.
Lattitude is an imaginary line which joins all the places which have the same angular distance north or south of the equator. It is measured in dfegrees. The Equator (00) is the longest line of lattitude known as the Great circle. It is equal to the circumference of the Earth. Other lines of lattitudes are of shorter lingth. The length of lines of lattitude decreases with the distance from the equator. All lines of lattitudes are circles and parallel to the Equator. Therefore, lines of lattitude are called parrallels of lattitude. There are 900 of North and South are points. Including equator totally there are 181 lattitudes on the globe. The ground distance between two degrees of lattitudes is 110.4 kms
IMPORTANT LATTITUDES :-
01. 00 lattitude-Equator or Great circle.
02. 23 ½0 North latitude – Tropic of cancer.
03.23 ½0 South latitude – Tropic of Capricorn.
04. 66 ½0 North latitude – Arctic Circle.
05. 66 ½0 South latitude – Arctic Circle.
06. 900 North – North pole.
07. 900 South – South pole.
The imaginary lines that intersect equator at right angle joining the north and south are called longitudes. On the globe, longitudes are shown as a series of semi-circle that run from pole to pole passing through the equator. All lines of longitude are of equal length. Lines of longitude are called Meridians ( ‘meri’-mid and ‘dian’-day ) because all places along the same meridian of longitude experience noon or mid-day at the same time.
The meridian passing through Greenwich (ENGLAND) has been chosen as Prime Meridian. It is marked as 00 longitude. There are 180 of longitudes to the east of Greenwich and 180 to the west. Thus there are 360 of longitudes. The zone between the Prime Meridian and 180 E longitude is called the Eastern Hemisphere. The opposite zone is called the Western Hemisphere.
LONGITUDE AND TIME :-
There is a definite relation between longitude and time. The earth is rotating on its axis and complete one rotation in 24 hours. This means 360 longitudes are covered in period of 24 hours. This gives rise to a time difference of 4 minutes for every one degree of longitude, 60 minutes or one hour for every 15 degree of longitude (360×4=1440/60=24 hours). The time difference is to be added ( E.G.A-East-Gain-Add ) in case of places to the east of GMT and in case of places to the west the time difference is to be substracted ( W.L.s-West-Lose-Substract ).
STANDARD TIME :-
To avoid confusion many countries follow uniform time throughout the country. Such uniform time is based on the central meridian of the country or the meridian on which the most important city is located. This uniform time which is followed throughout a country is called Standard Time of that country.
In India, 82 ½ 0 east longitude is considered as the Standard Meridian of the country. It passes through Allahabad of Uttara Pradesh. The time based on this Meridian is called the Indian Standard Time (IST). It is 5 hours and 30 minutes ahead of GMT.
INTERNATIONAL DATE LINE ( IDL ) :-
The circumnavigation of the world brought a new problem in keeping the correct date and day in the week for the travelers. Therefore a line passing through 1800 meridian diametrically opposite to the G.M.T was adopted as the point where circumnavigation should make adjustments. This line is supposed to pass Pacific ocean along the 1800 meridian but makes short detours in order to avoid land masses. This is known as the International Date Line because the date and day is changed whenever people cross this line by ships or airplanes.
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